1. Field of the Invention
The present invention relates to an alkaline earth aluminate phosphor for a cold cathode fluorescent lamp, which exhibits green emission with high luminance with emission in a blue wavelength region suppressed by ultraviolet rays having a wavelength of from 180 to 300 nm, with little decrease in emission luminance due to heat (heat deterioration), and a cold cathode fluorescent lamp which employs the above phosphor as a fluorescent layer, which has a high luminous flux and which realizes a beautiful display screen with a wide color reproduction range when used for a back light of e.g. a liquid crystal display.
2. Discussion of Background
In recent years, diffusion of flat panel displays (FPD) represented by liquid crystal displays (LCD) and plasma displays (PDP) is remarkable. FPD is classified into so-called light emitting displays such as PDP in which picture elements constituting an image on the panel themselves emit light, and non-light emitting displays such as LCD in which picture elements constituting an image on the panel themselves do not emit light and are combined with a back light. In LCD, an image is constituted on the panel by combination of a back light and a liquid crystal shutter, and a color filter is further combined to make color display of the image possible.
In recent years, the application of LCD is rapidly spreading to applications which requires color image display such as monitors and televisions, in addition to conventional applications to displays for personal computers. In such applications, it is very important to faithfully reproduce the color of the projected image, and the color reproduction range at least equal to a color cathode ray tube (CRT) is becoming necessary.
For a back light to be used for LCD, a cold cathode fluorescent lamp is mainly used, and in recent years, instead of a fluorescent lamp having a fluorescent layer comprising a single component phosphor of a halophosphate phosphor, a fluorescent lamp of three-wavelength type comprising as a fluorescent layer a phosphor with an emission spectrum having an intense peak with a narrow half value width in the vicinity of each of wavelength regions of 450, 540 and 610 nm, is rapidly spreading. However, as the phosphor for a three-wavelength type fluorescent lamp, particularly a green phosphor, a phosphor having an emission spectrum with agrees with the relative visibility intended to be used for lighting has been developed, and the phosphor developed to be used for lighting has been used as it is also for a cold cathode fluorescent lamp to be used for a back light of e.g. LCD. Accordingly, if the cold cathode fluorescent lamp is used for a back light of LCD as it is, even if it has a high luminous flux, the color reproduction range tends to be narrow, and if the thickness of the color filter of LCD is increased as a countermeasure, although the color reproduction range widens, the transmittance tends to be low, and luminance of LCD tends to decrease. Accordingly, development of a cold cathode fluorescent lamp which has a high luminous flux and with which the color reproduction range becomes wide when used for a back light of e.g. LCD has been desired.
Heretofore, as a green phosphor of a fluorescent lamp for lighting, a lanthanum phosphate phosphor (LAP phosphor) co-activated with trivalent cerium (Ce3+) and trivalent terbium (Tb3+) has been mainly used, however, JP-A-2001-228319 discloses that when a light source using as a fluorescent layer a bivalent manganese (Mn2+)-activated zinc silicate type phosphor (such as Zn2SiO4: Mn), a bivalent europium (Eu2+) and bivalent manganese (Mn2+)-co-activated barium magnesium aluminate type phosphor (such as BaMg2Al16O27: Eu, Mn) or a Mn2+-activated magnesium gallate type phosphor (such as MgGaO4: Mn), having an emission peak in a wavelength region of from 500 to 540 nm, is used as a back light of e.g. LCD, a beautiful display screen equal to a conventional color CRT which is bright and has a wide color reproduction range can be realized.
However, the Mn2+-activated zinc silicate type phosphor and the Mn2+-activated magnesium gallate type phosphor do not exhibit green emission with high luminance which can be practically used by ultraviolet rays by discharge of mercury, and if they are used as a fluorescent layer of a fluorescent lamp, the lamp luminous flux decreases with time due to reaction with mercury or ion impact, and thus they are not practically used yet.
On the other hand, the Eu2+ and Mn2+-co-activated alkaline earth aluminate phosphor exhibits green emission with high color purity highly efficiently by sensitization by Eu2+. This is because when excited by ultraviolet rays having a wavelength of from 180 to 300 nm, Eu2+ absorbs the ultraviolet rays and transfers the energy to Mn2+, whereby visible light is emitted. This phosphor has been often used practically as a phosphor of a fluorescent lamp for lighting or for copying machine, however, further development has been required since degree of heat deterioration of the phosphor is slightly high.
Further, JP-A-56-152883 proposes an aluminate phosphor comprising magnesium and barium and co-activated with Eu2+ and Mn2+, having a compositional formula of 0.7BaO.MgO.8Al2O3:0.05Eu, 0.2Mn for example, as a phosphor for copying machine. However, the phosphor has an emission spectrum having a first emission peak at a wavelength of 470 nm and a second emission peak at a wavelength of 510 nm when irradiated with ultraviolet rays, and when a cold cathode fluorescent lamp comprising this phosphor as a fluorescent layer is used as a back light of LCD, although the color reproduction range of green becomes wide, the color reproduction range of blue becomes narrow.
Further, JP-B-52-22836 discloses that an alkaline earth aluminate phosphor co-activated with Eu2+ and Mn2+, represented by the compositional formula Ba0.9Eu0.1Mg1.8Mn0.2Al16O27 for example, is useful as a phosphor for a fluorescent lamp. However, this phosphor tends to deteriorate in a baking step when a fluorescent layer is formed, and the luminous flux tends to decrease with time.
Still further, JP-A-4-304291 proposes a Eu2+ and Mn2+-co-activated barium magnesium aluminate phosphor as a phosphor to improve color rendering properties at the minimum sacrifice of brightness of a three-wavelength type fluorescent lamp. This phosphor exhibits emission having an emission spectrum having a first emission peak in a wavelength region of from 445 to 455 nm and a second emission peak in a wavelength region of from 510 to 520 nm, however, the intensity of the second emission peak is low as compared with the first emission peak, such being insufficient as a green phosphor.
Further, the phosphors as disclosed in JP-A-56-152883 and JP-B-52-22836 also exhibit emission with two peaks in the emission spectrum, however, the intensity of the second emission peak in a wavelength region of from 500 to 540 nm is low as compared with the first emission peak in the wavelength region of from 430 to 490 nm, such being insufficient as a green phosphor.
In FIG. 2, an emission spectrum (curve A) and a spectral transmission spectrum of a green filter (curve C) when a conventional Eu2+ and Mn2+-co-activated alkaline earth aluminate phosphor (conventional aluminate phosphor) having a compositional formula of {(Ba0.95Eu0.05).Mg0.99Mn0.01)O.5Al2O3} is excited by ultraviolet rays of 253.7 nm are shown. The intensity of emission in a wavelength region of from 430 to 490 nm is high as compared with the intensity of emission in a wavelength region of from 500 to 540 nm, and matching with the spectral transmission spectrum of a green filter is poor.
As mentioned above, when a cold cathode fluorescent lamp comprising as a fluorescent layer the conventional Eu2+ and Mn2+-co-activated alkaline earth aluminate phosphor is used as a back light of color LCD, the emission intensity in a blue wavelength region tends to be high as compared with the emission in a green wavelength region, and thus matching of emission of the phosphor with the spectral transmission spectrum of a green filter of the color LCD tends to be poor, and accordingly brightness and color purity of the color LCD tend to decrease.
Further, of the conventional alkaline earth aluminate phosphor, the crystal structure and the emission properties vary depending upon the composition proportion of the oxide of the alkaline earth and aluminum oxide constituting the matrix, and thus it is difficult to say that the above phosphor of a known composition is suitable for a fluorescent layer of a cold cathode fluorescent lamp.
Still further, such a Eu2+ and Mn2+-co-activated alkaline earth aluminate phosphor provides a lower emission luminance than the LAP phosphor which has conventionally been used, and a cold cathode fluorescent lamp using this phosphor as a fluorescent layer has a low luminous flux as compared with a cold cathode fluorescent lamp using the LAP phosphor as a fluorescent layer.
A cold cathode fluorescent lamp emits visible light by excitation of a phosphor by ultraviolet rays radiated from mercury, and is characterized in that the lamp tube diameter is smaller than that of a fluorescent lamp for lighting and the bulb wall load is high. Further, of a fluorescent lamp for lighting, the ultraviolet rays radiated from mercury are mainly ones having a wavelength of 253.7 nm, and influence of ultraviolet rays having a wavelength of 185 nm can be ignored, however, of a cold cathode fluorescent lamp, influence of ultraviolet rays having a wavelength of 185 nm, in addition to ultraviolet rays having a wavelength of 253.7 nm, can not be ignored. However, no studies have been disclosed regarding emission properties when the Eu2+ and Mn2+-co-activated alkaline earth aluminate phosphor is excited by ultraviolet rays having a wavelength of 185 nm. Further, in the cold cathode fluorescent lamp, the density of ultraviolet rays to be irradiated on the fluorescent screen tends to be high as compared with a fluorescent lamp for lighting, and the phosphor is likely to undergo luminance saturation. Thus, the activator concentration is preferably as high as possible within a range not causing concentration quenching, however, no studies have been disclosed regarding this point.